1. Field of the Invention
The present invention relates to an integrated circuit comprising a voltage generator and a circuit limiting the voltage supplied by the voltage generator.
2. Description of the Related Art
Booster circuits such as charge pumps allow an electric voltage higher than a determined supply voltage to be produced. In the field of integrated circuits, charge pumps for example are used to produce the high voltage Vpp for erasing and programming the floating-gate transistors of the electrically erasable and programmable memories (EEPROM, FLASH, FLASH-EEPROM . . . ).
FIG. 1 schematically represents a charge pump 1 supplying a boosted voltage Vpp to a load 2, here a capacitive load. This capacitive load is for example equal to the sum of the spurious gate capacitances of a plurality of floating-gate transistors to be erased or to be programmed simultaneously. The charge pump 1 is driven by clock signals S1, S2 in opposite phase delivered by an oscillator 3, and comprises a plurality of cascade-arranged pump stages the structure of which, well known by those skilled in the art, is not represented here. The charge pump 1 receives a supply voltage Vcc in the order of 2 to 5 volts at input. The amplitude of the voltage Vpp depends on the total number of cascade-arranged pump stages and is further proportional to the voltage Vcc.
The voltage Vpp is generally in the order of 10 to 20 volts and must not exceed a threshold Vppmax above which transistors to be erased or to be programmed could be damaged. Now, a charge pump is generally provided with a number of pump stages higher than the number of pump stages theoretically sufficient, so as to reduce the rise time of the voltage Vpp upon the activation of the charge pump. As a result, after a start period, the charge pump can deliver a voltage Vpp higher than the threshold Vppmax. Furthermore, the supply voltage Vcc can fluctuate considerably in relation to its nominal value taken into account at the time the charge pump is designed, and an increase in the voltage Vcc can lead to a corresponding increase in the voltage Vpp above the threshold Vppmax.
A control of the voltage Vpp must therefore be provided, so as not to exceed the threshold Vppmax.
As shown in FIG. 1, this control can be performed by a regulator 5 arranged at the output of the charge pump 1. The regulator 5 applies an on/off signal ON/OFF to the oscillator and stops the charge pump when the voltage Vpp reaches a pre-determined regulated value Vppreg, chosen lower than or equal to Vppmax, then restarts the pump when the voltage Vpp is below this value (on-off type regulation).
This solution is advantageous in terms of current consumption and flexibility of use, but a regulator has a relatively complex structure of a considerable size in terms of silicon surface occupied.
Another solution is to provide a simple voltage-limiting device at the output of the charge pump.
Thus, FIG. 2 represents a voltage-limiting device 6 comprising Zener diodes in series arranged between the output of the charge pump 1 and the ground, such as three Zener diodes 7, 8, 9 for example each having a threshold voltage in the order of 5V. The threshold voltage Vppmax of the voltage-limiting device is the sum of the threshold voltages of each of the diodes, such as 15V for example. This solution allows the active components of a regulator to be removed, but the Zener diodes themselves occupy a considerable surface area in an integrated circuit, and at least as large as the active components of a regulator. Furthermore, the manufacture thereof requires specific doping steps that penalise the cost price.
FIG. 3 represents a very simple solution in which the voltage Vpp is limited by a PN junction reverse arranged between the output of the charge pump and the ground and represented here in the shape of a diode 10. When the voltage Vpp reaches a value higher than the breakdown voltage of the PN junction, the latter becomes on by avalanche effect. The number of diodes to be provided in series depends on their breakdown voltage and on the maximum value Vppmax sought. As a reverse-biased PN junction generally has a breakdown voltage in the order of 17V, a single diode can allow the voltage Vpp applied to memory cells to be limited between 15 and 20V.
In a MOS or CMOS integrated circuit, the diode 10 is, in reality, a diode-arranged MOS transistor, that is a transistor having its gate linked to its drain or to its source (depending on whether it is a PMOS or NMOS transistor). Now, a diode transistor has the disadvantage that its breakdown voltage varies with time, since electric charges supplied by the avalanche current are trapped in the gate oxide of the diode transistor. This phenomenon is generally designated junction “Roll-off”, and occurs after quite a small number of breakdowns, generally below 100, since the limiting current that passes through the diode transistor is often quite high, from several tens to several hundreds of microamperes.
Certain manufacturers of integrated circuits adapt to this spurious effect but it results in a substantial imprecision as far as the voltage for triggering the PN junction limiting devices is concerned, which tends to increase with time. Now, it is likely that this imprecision is the cause of the mediocre service life of certain memories in integrated circuits, the memory cells of which receive voltage Vpp peaks that are increasingly higher as the degradation of the breakdown voltage of the PN junction of their voltage-limiting devices increases.